1. Field of the Invention
The present invention relates to an automatic impedance adjusting apparatus for a microwave load and an automatic impedance adjusting method therefor, more particularly, to an automatic impedance adjusting apparatus for adjusting an impedance seen looking toward a microwave load at a point of a microwave transmission line to a desirable impedance such as an impedance of a microwave oscillator, and an automatic impedance adjusting method
2. Description of Related Art
FIG. 1 shows a conventional automatic microwave impedance matching apparatus proposed in the Japanese patent laid open publication No. (JP-A) 63-15502/1988.
Referring to FIG. 1, a rectangular waveguide 100 of the automatic impedance matching apparatus is connected between a microwave oscillator and a microwave load. On the microwave oscillator side in the rectangular waveguide 100, there is arranged a voltage standing wave detector composed of five probes PR11 to PR15 therein aligned at an equal distance of .lambda.g/8 in the longitudinal direction thereof, wherein .lambda.g is an average waveguide length of the microwave propagating in the rectangular waveguide 100. On the microwave load side in the rectangular waveguide 100, two pairs of composite stubs ST1 and ST2 are arranged at different positions in the longitudinal direction thereof.
The first composite stub ST1 is composed of two stubs S11 and S12 mounted at both ends of a seesaw rod, and the stubs S11 and S12 are driven by a stub driving motor M11 so as to be inserted into and drawn out from the rectangular waveguide 100 reciprocally by a seesaw motion of the seesaw rod. On the other hand, the second composite stubs ST2 is composed of two stubs S13 and S14 mounted at both ends of another seesaw rod, and the stubs S13 and S14 are driven by another stub driving motor M12 in the same manner as the stubs S11 and S12 of the first composite stub ST1.
A voltage standing wave of the microwave propagating in the rectangular waveguide 100 is detected by diodes DI11 to DI15 connected to the probes PR11 to PR15, respectively. After the output of the diode DI11 is outputted to the anode of the diode DI15 so as to compose the output of the diode DI15 therewith, the composed output is inputted to an input terminal of a differential amplifier AMP11 through a resistor R11. Each output of the diodes DI12 and DI14 is inputted to each input terminal of a differential amplifier AMP12, and the output of the diode DI13 is inputted to another input terminal of the differential amplifier AMP11.
The output of the differential amplifier AMP11 is outputted to the stub driving motor M11 through a power amplifier AMP21, and the output of the differential amplifier AMP12 is outputted to the stub driving motor M12 through a power amplifier AMP22.
In the automatic microwave impedance matching apparatus constructed above, output voltages Va.sub.11 and Va.sub.12 of respective differential amplifiers AMP11 and AMP12 are expressed by the following equations with voltages Vp.sub.11 to Vp.sub.15 of the voltage standing wave detected by respective probes PR11 to PR15. EQU Va.sub.11 =Vp.sub.11 -Vp.sub.14 ( 1) EQU Va.sub.12 =1/2(Vp.sub.11 +Vp.sub.15)-Vp.sub.13 ( 2)
When the stub driving motors M11 and M12 are driven according to the output voltages Va.sub.11 and Va.sub.12, respectively, the voltage standing wave in the rectangular waveguide 100 changes, namely, an impedance seen looking toward the load at the voltage standing wave detector changes. Since the probes PR11 to PR15 are arranged at an equal distance of .lambda.g/8 in the longitudinal direction of the rectangular waveguide 100, the output voltages Va.sub.11 and Va.sub.12 of respective differential amplifiers AMP11 and AMP12 are orthogonal to each other. Therefore, in the feed back system of the automatic impedance matching apparatus, the composite stubs ST1 and ST2 are driven by the stub driving motor M11 and M12 so that each of the output voltages Va.sub.11 and Va.sub.12 becomes zero. When both the output voltages Va.sub.11 and Va.sub.12 become zero, the impedance of the microwave oscillator is matched to the load impedance.
However, when the above automatic microwave impedance matching apparatus is applied to an apparatus comprising a plasma generating apparatus such as a plasma etching apparatus, a plasma CVD apparatus or the like, the following problems are caused.
(1) A state of a plasma generated by the plasma generating apparatus may change suddenly, and then, a load impedance thereof may change. In this case, the conventional automatic impedance matching apparatus can not track the change in the load impedance thereof accurately, resulting in a hunting phenomenon therein.
(2) As shown in FIG. 2, there is a hysteresis in a relationship between an output power of the microwave oscillator and a load impedance of the plasma generating apparatus, and particularly, the hysteresis has two discontinuous points 101 and 102. Therefore, the load impedance changes discontinuously at respective discontinuous points 101 and 102, and then, the automatic impedance matching apparatus can not match the load impedance to the impedance of the microwave oscillator.
It is known to those skilled in the art that a plasma may be generated more stably in a state slightly shifted from the impedance matching state. Therefore, it has been desired that the impedance seen looking toward the load is automatically adjusted to a desirable impedance. However, the automatic microwave impedance matching apparatus can not adjust the impedance seen looking toward the load to a desirable impedance.